I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for detecting one or more pilot signals with a programmable matched filter searcher.
II. Description of the Related Art
Pseudorandom noise (PN) sequences are commonly used in direct sequence spread spectrum communication systems such as that described in the IS-95 over the air interface standard and its derivatives such as IS-95-A and ANSI J-STD-008 (referred to hereafter collectively as the IS-95 standard) promulgated by the Telecommunication Industry Association (TIA) and used primarily within cellular telecommunications systems. The IS-95 standard incorporates code division multiple access (CDMA) signal modulation techniques to conduct multiple communications simultaneously over the same RF bandwidth. When combined with comprehensive power control, conducting multiple communications over the same bandwidth increases the total number of calls and other communications that can be conducted in a wireless communication system by, among other things, increasing the frequency reuse in comparison to other wireless telecommunication technologies. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both of which are assigned to the assignee of the present invention and incorporated by reference herein.
FIG. 1 provides a highly simplified illustration of a cellular telephone system configured in accordance with the use of the IS-95 standard. During operation, a set of subscriber units 10a-d conduct wireless communication by establishing one or more RF interfaces with one or more base stations 12a-d using CDMA modulated RF signals. Each RF interface between a base station 12 and a subscriber unit 10 is comprised of a forward link signal transmitted from the base station 12, and a reverse link signal transmitted from the subscriber unit. Using these RF interfaces, a communication with another user is generally conducted by way of mobile telephone switching office (MTSO) 14 and public switch telephone network (PSTN) 16. The links between base stations 12, MTSO 14 and PSTN 16 are usually formed via wire line connections, although the use of additional RF or microwave links is also known.
Each subscriber unit 10 communicates with one or more base stations 12 by utilizing a rake receiver. A RAKE receiver is described in U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated herein by reference. A rake receiver is typically made up of one or more searchers for locating direct and multipath pilot from neighboring base stations, and two or more fingers for receiving and combining information signals from those base stations. Searchers are described in co-pending U.S. patent application Ser. No. 08/316,177, entitled xe2x80x9cMULTIPATH SEARCH PROCESSOR FOR SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMSxe2x80x9d, filed Sep. 30, 1994, assigned to the assignee of the present invention and incorporated herein by reference.
Inherent in the design of direct sequence spread spectrum communication systems is the requirement that a receiver must align its PN sequences to those of the base station. In IS-95, each base station and subscriber unit uses the exact same PN sequences. A base station distinguishes itself from other base stations by inserting a unique offset in the generation of its PN sequences. In IS-95 systems, all base stations are offset by an integer multiple of 64 chips. A subscriber unit communicates with a base station by assigning at least one finger to that base station. An assigned finger must insert the appropriate offset into its PN sequence in order to communicate with that base station. It is also possible to differentiate base stations by using unique PN sequences for each rather than offsets of the same PN sequence. In this case, fingers would adjust their PN generators to produce the appropriate PN sequence for the base station to which it is assigned.
Subscriber units locate base stations by utilizing searchers. A fast, flexible, and hardware efficient matched filter searcher is described in co-pending U.S. patent application Ser. No. 09/283,010 (hereinafter the ""010 application), entitled xe2x80x9cPROGRAMMABLE MATCHED FILTER SEARCHERxe2x80x9d, filed Mar. 31,1999, assigned to the assignee of the present invention and incorporated herein by reference. This searcher adds flexibility to the parallel computation features of a matched filter, allowing a variable number of coherent accumulations and a variable number of non-coherent accumulations to be performed at high speed for a wide range of search hypotheses in a resource efficient manner. Many of the features of this searcher are applicable to the present invention as well, and will be described in greater detail below.
The FCC has mandated that by October 2001, carriers must provide the location of a cell phone user making an emergency 911 call to within 125 meters. In addition to providing mandated location services, wireless carriers are interested in providing revenue-generating location-based services such as roadside assistance, traffic updates, yellow page directory assistance, and the like.
A variety of approaches can be taken to solve this problem, among them are solutions based on the Global Positioning System (GPS). The Global Positioning System comprises a constellation of 24 satellites. Each satellite contains a clock that is kept synchronized to GPS time by monitoring ground stations. GPS receivers on the ground can use signals received from several GPS satellites to determine position and time.
Each GPS satellite transmits two microwave carriers: a 1575.42 MHz L1 carrier which carries the signals used for Standard Positioning Service (SPS), and a 1227.60 MHz L2 carrier which carries signals needed for Precise Positioning Service (PPS). PPS is used by governmental agencies and allows a higher degree of accuracy in positioning.
The L1 carrier is modulated by the Coarse Acquisition (C/A) code, a 1023-chip pseudorandom code transmitted at 1.023 Mcps that is used for civil position location services. Each GPS satellite has its own C/A code that repeats every 1 ms. The code used for PPS is a 10.23 MHz code that is 267 days in length.
Each GPS satellite has a different C/A code that belongs to a family of codes called Gold codes. Gold codes are used because the cross-correlation between them is small. Each GPS satellite generates a unique C/A code sequence. A GPS receiver reproduces the C/A sequence for a particular satellite and correlates it with the received signal over all possible offsets. When correlation is found, the start time of the code is referred to as the time of arrival (TOA) at the receiver. This TOA is a measure of the range to the satellite, with an offset due to any mismatch between the receiver clock and GPS time. The TOA is also referred to as the pseudorange. Once the pseudoranges from each of 4 satellites have been obtained, a position fix can be computed by solving for the intersection of 4 spheres. Using 4 satellites allows the receiver clock uncertainty to be cancelled out.
GPS position location can be done based solely on signals received from GPS satellites, as just described, but can also be accomplished using a hybrid scheme. Such hybrid schemes are often useful when additional information is available to reduce the complexity of the position location task. One example is a wireless network, where the base station can provide information to limit the required search windows or can provide accurate time corresponding to GPS time. One such system is described in co-pending U.S. patent application Ser. No. 09/187,939, entitled xe2x80x9cMOBILE COMMUNICATION SYSTEM WITH POSITION DETECTION TO FACILITATE HARD HANDOFFxe2x80x9d, filed Nov. 6, 1998, assigned to the assignee of the present invention and incorporated herein by reference.
There is commonality in the hardware needed to search for CDMA pilots (or the like) and that needed for position location (whether pure GPS or some hybrid). However, in contrast to the single pilot code systems such as that described in IS-95, multiple pilot code systems such as GPS would inherently benefit from the ability to search more than one pilot code simultaneously. Many devices, such as subscriber unit 10 in FIG. 1, will need to perform both types of searching during their normal course of operation. There is a need in the art for a fast, flexible, hardware-efficient searcher that combines CDMA pilot searching with multiple pilot searching for systems such as GPS position location.
A novel and improved method and apparatus for searching is described. This searcher combines the ability to search multiple offsets of single pilots, such as those found in the IS-95 system, with the ability to search multiple pilots, such as those found in a GPS location determination system. Both types of searching can be done in a single architecture combining the parallel computation features of a matched filter with the flexibility of allowing a variable number of coherent accumulations and a variable number of non-coherent accumulations to be performed at high speed for a wide range of search hypotheses in a resource efficient manner. This invention allows for parallel use of the matched filter structure in a time-sliced as well as space-sliced manner to search multiple windows. In addition, the searcher allows for optional independent Walsh decovering for each search window. Both approaches allow for optional frequency searching of any offset.
I and Q channel data are despread utilizing a matched filter structure. The matched filter structure can be configured as one large matched filter with a single I/Q data input, or it can be configured to accept a plurality of signals, essentially breaking the matched filter into a plurality of smaller matched filters (this essentially describes space-slicing). The plurality of inputs can be independent signals from a variety of sources, such as multiple satellites in a GPS network.
The in-phase and quadrature amplitudes from the matched filter are delivered to coherent accumulators to sum for a programmable duration of time. This coherent accumulation can occur for the entire matched filter structure, or multiple accumulations can be generated based on the subsets of the matched filter associated with each of a plurality of input signals. These coherent accumulations are available for further processing in a device such as a DSP. For single pilot searching, the coherent amplitude accumulations are squared and summed to produce an energy measurement. The energy measurement is accumulated for a second programmable time to perform non-coherent accumulation. The resulting value is used to determine the likelihood of a pilot signal at that offset.
Each matched filter structure comprises an N-value shift register for receiving data, a programmable bank of taps to perform despreading and optional Walsh decovering, and an adder structure to sum the resulting filter tap calculations. The matched filter structure can optionally be used in a time-sharing manner to search multiple windows as dictated by a multiplexor which supplies various streams of tap values for despreading (with optional Walsh decovering included in the tap values). In addition, an optional phase rotator can be added to apply multiplexed phase values to perform frequency searching. Every cycle the matched filter structure produces an intermediate calculation for a particular offset (with optional Walsh decovering and optional phase rotation) which includes N calculations based on the data in the shift register.